I have my heads off and there is a nice 1/16 layer of carbon build up. Can I just spray my pistons with carb cleaner? What effect will the carb cleaner have on running down the cylender and by the rings? any other way to clean them?

 

If it's not heavy oil-saturated carbon then you can carefully scrape the worst of it off while vacuuming with a shop vac to immediately collect the particles as they break free. The problem is that the carbon very easily falls in the crevice between the piston and bore where it can quickly ruin the rings upon restart.

My preferred method of cleaning out carbon is to run a drizzle of water through the engine as it is running at about 2000 RPMs. It basically "steam cleans" the combustion chambers and pistons as the motor runs. Spend a good 10-15 minutes running a quart of water through the motor. Change oil immediately afterwords. You'll see the carbon blowing out the tailpipes as you do this. It's an old trick but it still works.